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PFAS Total Oxidisable Precursor Assay (TOP Assay)

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A Powerful Tool to Estimate the PFAS Risk Potential

Apr. 2019. Not only since the toxicological re-evaluation of tolerable intake levels for the two most important representatives of per- and polyfluorinated alkyl substances (PFAS) (perfluoroctane sulfonate (PFOS) and perfluoroctanoic acid (PFOA)) by the European Food Safety Authority, EFSA, this group of environmental contaminants is under the focus of risk managers, environmental authorities, official food and feed monitoring and producers.

A novel method based on Houtz and Sedlak (Environ. Sci. Technol. 46 (2012) 9342–9349) which provides more comprehensive understanding of the risk potential concerning PFAS present in the environment is available now. This so called TOP assay can help to determine the concentration of innumerable PFAS that cannot be covered by conventional analytical methods.

TOP (Total Oxidisable Precursor) Assay

Current conventional methods for PFAS analysis allow to analyse a list of around 30 different compounds. There are, however, much more PFAS left undetermined, which might be of (high) risk and can be converted into the persistent perfluorinated alkyl substances (e.g. perfluorooctanoic acid, PFOA) under certain conditions e.g. in the environment. The TOP assay is a hydroxyl radical based oxidation reaction. Precursors are transformed to dead end perfluoroalkylic acids (PFAAs) in such reactions. The evaluation of pre- and post-TOP assay data may therefore offer a clearer view of PFAS being present in specific samples

Real Example at Eurofins:
Theoretical Proof in Experiments with PFAS Added to Water

Several PFAS precursors were added to water and their concentrations were measured before and after TOP-Assay. The data provides theoretical proof for TOP assay. As follows you will find two examples:

: : Water with addition of H2PFDA, vbefore and after the TOP assay

Green line: Water with addition of H2PFDA added after TOP assay - PFBA, PFPeA, PFHxA, PFHpA and PFOA, but not H2PFDA are detected
Red line: Water with addition of H2PFDA without TOP-assay

Water with addition of N-MeFOSAA and N-EtFOSAA, before and after the TOP-assay

Green line:Water with addition of N-MeFOSAA and N-EtFOSAA after TOP assay - only PFOA, but not N-MeFOSAA und N-EtFOSAA are detected
Red line: Water with addition of N-MeFOSAA and N-EtFOSAA without TOP assay

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